The hydrogen-oxygen fuel cell receives attention as a power generating system having little adverse effect on the global environment because in principle, its reaction product is water only. Solid polymer electrolyte fuel cells were once mounted on spaceships in the Gemini project and the Biosatellite project, but their power densities at the time were low. Later, more efficient alkaline fuel cells were developed and have dominated the fuel cell applications in space including space shuttles in current use.
Meanwhile, with the recent technological progress, solid polymer fuel cells are drawing attention again for the following two reasons: (1) the development of highly ion-conductive membranes for use as solid polymer electrolytes and (2) the impartment of high activity to the catalysts for use in gas diffusion electrodes by the use of carbon as the support and an ion exchange resin coating.
For improved performance and low cost, the electric resistance can be reduced through reduction in thickness of a solid polymer electrolyte membrane. Solid polymer electrolyte membranes, which are usually obtained by using a polymer having sulfonic acid groups, can be reduced in thickness by the following two methods. (1) Heat extrusion of a polymer having precursors of sulfonic acid groups (SO2F groups or SO2Cl groups) into a thin membrane, followed by hydrolysis of the precursors and conversions of the precursors into the acid form by acid treatment. (2) Casting of a uniform dispersion of a polymer having sulfonic acid groups in a dispersion medium such as an alcohol on a support such as a polyester film followed by drying.
However, the method (1) has a minimum limit on the thickness of obtainable membranes because thin membranes are difficult to handle during hydrolysis and acid treatment. Further, because the hydrolysis and the acid treatment cannot be carried out successively at a high rate, the method is disadvantageous in terms of cost. On the other hand, the method (2) is advantageous in terms of cost because there is no minimum limit on the thickness of obtainable membranes, and the polymer can be subjected to the hydrolysis and the acid treatment in large batches before the uniform dispersion is cast into membranes.
Accordingly, use of a thin membrane obtained by casting as in (2) in a membrane-electrode assembly for solid polymer electrolyte fuel cells was proposed (JP-A-6-44982). However, thin membranes obtained by this method have defects such as low strength, vulnerability to cracking, and change in swelling in water or steam with time, presumably, though not precisely, because the dispersion of the polymer having sulfonic acid groups has a structure having micelles of the polymer dispersed in the dispersion (JP-B-61-40267).
In order to solve these defects, addition of triethyl phosphate, dimethyl sufoxide or 2-ethoxyethanol to a polymer dispersion (JP-A-61-40267) and addition of N,N-dimethylformamide or ethylene glycol were proposed (Anal. Chem., 58, 2570 (1986)).
However, in these methods, because a solvent having a relatively high boiling point is added, a residue of the solvent remains in a membrane cast at a low temperature, or casting requires a long time or a high temperature not to leave a residue of the solvent. Accordingly, there is a problem with production efficiency.
Another possible approach is mere heat treatment at a temperature higher than the glass transition temperature of an ion exchange polymer without addition of a solvent having a high boiling point, and heat treatment of a membrane-electrode assembly made of an electrolyte membrane and electrodes bonded together at a temperature of from 130 to 270° C. was proposed (Japanese Patent 2781630). However, there was a problem that burning of a residual solvent which can occur at an elevated temperature in the presence of the catalyst deteriorates the performance.